(1) Field of the Invention:
The present invention relates to pneumatic tires. More particularly, the invention relates to pneumatic tires which greatly improve driving performance, braking performance and cornering stability on ice and snow roads without deteriorating cornering stability, durability and wear resistance in the summer.
(2) Related Art Statement:
In order to assure driving performance, braking performance and cornering stability on roads with on ice and snow formerly, spike tires in which spike pins are piled into a surface portion of a tread have frequently been used. However, such tires pose grave social problems because dust is produced due to wearing of the spike pins themselves and wearing or damages of the roads. To cope with this, a projecting amount of the spike pin and the number of spike pins per unit area have been controlled and the material of the spike pins has been examined. However, no fundamental solution of such social problems has been realized.
On the other hand, studless tires exhibiting driving performance, braking performance and cornering stability on roads with ice and snow without using spikes or chains have been recently examined and rapidly widely used, but it cannot necessarily be said that performances on road with ice and snow is exhibited as compared with the spike tires.
With respect to the rubber for the studless tires, a rubber composition using a polymer having a low glass transition point is used to ensure elasticity of the rubber at low temperature. However, depending upon a hysteresis characteristic of such a polymer, it remains a problem in that although the tire performance is exhibited to some extent in an ice and snow temperature range, braking performance and cornering stability are not sufficient on wet roads or dry roads.
Further, as disclosed in Japanese patent application Laid-open Nos. 55-135,149, 58-199,203 and 60-137,945, it is known that when a rubber composition compounded with a large amount of a softener or a plasticizer is used as a tread rubber, the performance at low temperatures is similarly improved. However, these techniques have serious problems in that although the rate at which the performance on ice and snow roads is improved, reduction in wear resistance and separation of the tread rubber occur during running on general roads.
Although each of the above techniques affords excellent performance on roads with ice and snow in a relatively low temperature range (a so-called dry-on-ice state of) not more than -5.degree. C., a sufficient coefficient of friction cannot be obtained on ice and snow in a wet state (a so-called wet-on-ice state) near 0.degree. C. Therefore, it cannot be said that driving performance, braking performance and cornering stability are improved over the entire range.